LAB-RADIOBIOLOGY
Authors: S. R. Floyd, M. E. Pacold, S. M. Clarke, E. Blake, A. Fydrych, R. Ho, M. J. Lee, D. E. Root, A. E. Carpenter, D. M. Sabatini, C. A. French, J. E. Bradner, C. C. Chen, M. B. Yaffe, E. Le Rhun, F. Massin, A. Lefevre, J. Bonneterre, M. d. C. Bittencourt, G. Faure, R. Hiramatsu, S. Kawabata, Y. Yamada, S.-I. Miyatake, T. Kuroiwa, S. Li, A. P. Chou, W. Chen, R. Chen, Y. Deng, H. S. Phillips, K. F. Faull, T. Cloughesy, L. M. Liau, A. Lai, K. Mori, R. Ishikura, Y. Tomogane, S. Izumoto, N. Arita, J. Piao, G. Auyeung, E. Policarpio, V. Tabar, T. P. C. Yeung, L. Morrison, L. Hoffman, T.-Y. Lee, G. Bauman, S. Yartsev, S. Ryu, A. Kolozsvary, M. Lapanowski, K. Jenrow, S. Brown, J. H. Kim, R. J. Brown, J. Love, D. Warburton, W. McBride, S. Bluml, X. Ren, B. Vanderwaal, J. Jaboin, A. L. Baldock, S. Anh, R. Rockne, M. Neal, K. Clark-Swanson, G. Sterin, A. D. Trister, H. Malone, V. Ebiana, A. M. Sonabend, M. Mrugala, J. K. Rockhill, D. L. Silbergeld, A. Lai, T. Cloughesy, G. M. McKhann, J. N. Bruce, R. Rostomily, P. Canoll, K. R. Swanson, A. Hawkins-Daarud, A. Baldock, C. Bridge, D. Corwin, J. K. Rockhill, M. M. Mrugala, R. Rockne, K. R. Swanson, A. L. Baldock, K. Yagle, S. Anh, D. Born, P. Swanson, R. Rockne, K. R. Swanson, A. Hawkins-Daarud, R. Rockne, K. R. Swanson
Journal: Neuro-Oncology (2012)

INTRODUCTION: Radiation therapy (RT) is a critical part of the treatment regimen for glioblastoma.However, tumor recurrence after RT is nearly universal.The mechanism of resistance remains elusive.We conducted an RNAi screen to identify the genetic basis of therapeutic failure after RT.METHODS: We investigated the role of chromatin structure glioblastoma therapeutic response via an RNAi screen of 1002 shRNAs directed against 196 genes with known and predicted chromatin-modifying activity for their involvement in DNA damage response.Mechanistic and pathologic correlative experiments were performed using the top candidate.RESULTS: The screen revealed that Brd4, a double bromodomain-containing protein, was a top candidate in the screen.Brd4 knockdown cells displayed altered chromatin structure and enhanced survival after RT, whereas overexpression of Brd4 isoform B resulted in enhanced RT-induced lethality.Further, loss of Brd4 resulted in an increase in both the number and size of RT-induced gH2AX nuclear foci, whereas overexpression of Brd4 isoform B completely suppressed gH2AX formation, despite equivalent double-strand break formation.These results suggest that BRD4 functions as an endogenous inhibitor of chromatin condensation.We showed that this inhibition is mediated through physical interaction with and inhibition of the SMC2 condensin complex.Importantly, high expression of Brd4 isoform B correlated specifically with response to RT in independent specimen collections (p ¼ 0.01).Further, expression of Brd4 isoform B was significantly reduced in recurrent glioblastoma when compared to that in newly diagnosed glioblastoma (p ¼ 0.0003).CONCLUSIONS: These data implicate Brd4 as an endogenous insulator of DNA damage signaling and that expression of Brd4 in glioblastoma can modulate clinical response to DNA-damaging cancer therapy.This finding, along with the recently published driver mutation in the Histone 3.3 gene in pediatric glioblastomas, underscores the importance of chromatin modification in glioblastoma pathogenesis and etiology.